Verified answer

tg(x) = sen(x)/cos(x) = b

sen(x) = b*cos(x)

sen²(x) + cos²(x) = 1

b²*cos²(x) + cos²(x) = 1

cos²(x)*(b + 1) = 1

cos²(x) = 1/(b + 1)

pronto

tg x= b

tg x = sen x/cos x

b.cos x = sen x

sen²x+cox²x=1

(b.cos x)²+cos²x=1

b².cos²x+cos²x=1

cos²x(b²+1)=1

cos²x= 1/b²+1